Microsoft Security Essentials. I'm serious, it's pretty good (assuming your machine was clean to begin with).
Another thumbs up for Microsoft Security Essentials.
http://windows.microsoft.com/en-US/windows/products/security-essentialsEvery now and then you can run the Microsoft Malicious software Removal Tool in full scan mode (type "mrt" in the run menu on any XP/Vista/Win7 system that is up to date with the Windows Updates.) "mrt" runs regularly in the background after a standard Windows update, but only in a fast mode rather then the full mode.
If you need to do an offline sweep for rootkits and other tougher viruses, you can get Microsoft "Windows Defender Offline", burn it to a disk, and then boot off the disk. This scans without running any program on the hard disk, so a rootkit cannot hide itself the way it can under Windows.
http://windows.microsoft.com/en-US/windows/what-is-windows-defender-offline.If you find you cannot do Windows Updates, you cannot run "mrt" and Security Essentials does not seem to run properly, then your computer is almost certainly infected with viruses. Windows Defender Offline will always run, no matter how many viruses are affecting the computer.
Back when I was using AVG, I was regularly annoyed by the way they kept updating the versions so you had to download and install the new version as a separate task, and each time, they tried to hide the free version to get people to go to the paid version instead. If you are looking after a number of PC's, it is just annoying that the standard update does not keep you updated to the latest version.
Richard.